This invention relates to an apparatus for unidirectional solidifications of components such as turbine blades and vanes.
Apparatus for directional solidification of turbine parts are known in the art and comprise an open bottom hot ceramic mold containing molten alloy mounted on a water cooled copper chill block and pulled out of a mold heater in a controlled manner. The casting solidifies from the top of the chill-block (i.e. mould bottom) unidirectionally upward by heat conduction into the chill-block via the solidified part of the casting initially and later by radiative heat transfer from the mold-wall to the water cooled furnace-wall as the resistance against heat conduction to the chill-block through the increasing length of solidified part goes above that of heat conduction from solidifying casting to mold wall and radiative of heat transfer from mold wall to the water-cooled furnace wall. A variety of equipments designs are known ranging from large capacity furnaces employing as much as 50 kg of superalloy melting crucible and large molds of about 150 mm diameter and 600 mm height to furnace designs for casting in molds as small as 140 mm diameter and height about 200 mm. Large sized furnaces offer higher production rate but poor casting quality owing to low temperature gradient across the solid liquid interface compared to those with relatively smaller mold heaters. Higher processing temperatures in large sized furnaces provided an improved temperature gradient, but result in poor casting quality due to increased melt-mold reactions or else would require costly molding systems to avoid or reduce melt-mold reactions. Smaller furnace designs on the other hand lead to better quality castings but for small to medium sizes only. Adequate mould rigidity against metallostatic pressure in the case of large and tall components requires increasingly thick mold wall (in proportion to mold height) which in turn deteriorates the temperature gradient across the solid liquid interface of the solidifying casting. Moreover such furnaces of the prior art often do not have precise control over the crystallographic orientation of single crystal components since the crystal generally emerges here through random nucleation and grain growth competition in helical grain selectors or other geometrical constructions. In the furnace of U.S. Pat. No. 4,469,161, seed crystal of desired crystallographic orientation is placed to the mold bottom so that the same seed texture will extend into the component as solidification proceeds from the bottom to the top of the mold cavity. The casting yield however, in such case has been only marginal since the said crystal surface is subjected to an aggressive atmosphere during mold heating due to vaporisation of volatile compounds and also to oxidation in the dynamic vacuum at high temperature (xe2x88x92155xc2x0 C.). All these factors increase the risk of polycrystalline growth.
Better control of crystal orientation through seed implantation can be achieved if the seed crystal enters the mold after heating the mold to the desired temperature just at the event of melt pouring into the mold. European Patent no 0496978 A1 and U.S. Pat. No. 5,261,480 disclose a seeding method for single crystal component casting using a complex arrangement of splitting the furnace into four or more separate chambers employing several vacuum interlocks and transporting devices for loading, unloading and heating of remelt bar, mold and seed crystal in isolation from one another and then simultaneously bringing melt (by tilt pouring) and seed crystal (by quick movement of seed carriages from lower to upper chamber and actuating a clamping device to hold the mold tightly against any melt leakage) and simultaneously inserting the seed crystals into the mold. All these require precision movement of various parts with faithful interlocking, sequencing and quick start-stop movements with positional and durational accuracy and repeatability in vacuum at high temperature avoiding any impact between various parts or melt leakage where occurrence of refractory debris with time and melt droplets due to melt pouring is unavoidable.
An object of this invention is to propose an apparatus for producing columnar grained as well as single crystal castings over a wide range of shapes and sizes (from about 50 mm to about 500 mm length) with very high casting quality and productivity.
Another object of this invention is to propose an apparatus with minimum vacuum sealing joints in order to minimise leak rate and to rely upon very few mechanisms involving very few moving or sliding parts and achieve thereby greater reliability, operational case and less maintenance relative to the apparatus of the prior art.
Yet another object of the invention is to propose an apparatus for casting large and tall components in much thinner molds in order to improves metallurgical quality and productivity employing much less induction melting capacity than what the furnaces of the prior art require.
According to this invention there is provided an apparatus for producing unidirectionally solidified components over wide range of shapes and sizes from the smallest aero-engine turbine parts to the largest industrial gas turbine parts with improved quality and productivity comprising at least a first unit having;
(i) at least one melting and casting chamber having mold heating station, an induction melting coil and an intermediate melt pouring funnel indexing device;
(ii) at least one mold charging compartment for said casting chamber, a movable chill block disposed within said mold charging compartment, a seed crystal within said chill-block, an actuator for-raising and lowering the chill block, a seed crystal ejector coaxially disposed within said actuator and so as to cause a displacement of the seed crystal independent to that of the chill block;
(iii) at least one alloy-bar-stock charging compartment;
(iv) an alloy bar stock melting crucible adapted to be disposed within said casting chamber;
(v) a vacuum pumping system for the melting and casting chamber;
(vi) a mechanical vacuum pump for the aloy bar stock and molding charging compartments.
The apparatus in accordance with one embodiment of the present invention comprises at least a first unit having a single melting and casting chamber with an alloy bar stock compartment and mold charging chamber at the chamber top and bottom via small isolation valves. For efficient production of single crystal components with desired crystal orientation, a heat conductive chill block that clamps and carries the ceramic mold is provided with an ejector to insert seed crystals into the preheated mold at the instant of melt pouring. A hot zone for mold heating is designed for precise control of solid-liquid interface position and desired level of temperature gradient at the interface employing an optimum aperture of radiation baffle to accomplish defect free unidirectional freezing at a minimum hot zone temperature. The apparatus of this invention is capable of casting either one component at a time or several components clustered together in the ceramic mold depending upon the component cross section. Unidirectional solidification for tall components having large cross-sections in relatively thin walled molds takes place because of its intermediate melt pouring funnel indexing device for melt pouring in instalments. An added advantage of this arrangement is the scope to make larger castings than the induction melting capacity with the concept of quick melting in open-bottom small-crucibles and quiet pouring of clean melt therefrom.
In accordance with another embodiment several such first units are radially connected via isolation valves to a central diffusion pump of a single vacuum pumping system. A separate roots blower-mechanical vacuum pump combination is provided for the alloy barstock and mold charging compartments to ensure increased production and efficient machine utilisation.
In accordance with the yet another embodiment for unidirectional solidification of components of large size both in height, cross-section, and casting thickness, a low melting liquid metal bath instead of seed crystal is provided under the radiation baffle of the mold heating hot zone for efficient heat transfer and high temperature gradient across the solid-liquid interface of the solidifying casting. The chill block arrangement along with mold charging compartment is placed at the top of melting and casting chamber in place of alloy bar stock charging compartment that is displaced to the periphery.
In accordance with another embodiment, an apparatus is provided including at least one of the first unit described in the foregoing and a second unit having a casting chamber with an induction melting coil horizontally retractable and its mold charging compartment disposed at the chamber top, adjacent to the alloy charging compartment.